For the first time in nearly 600 years, a pope is resigning from his post as leader of the Roman Catholic Church.

Pope Benedict XVI announced Monday morning that he is stepping down effective Feb. 28.

From Rome, NPR's Sylvia Poggioli tells the NPR Newscast that the pope is citing his advanced age (85) and diminishing strength. Sylvia says "he had been thinking about this for a long time" and appears to have "decided to do it for the good of the church."

"Announced his decision in Latin during a meeting of Vatican cardinals Monday morning.

" 'After having repeatedly examined my conscience before God, I have come to the certainty that my strengths due to an advanced age are no longer suited to an adequate exercise of the Petrine ministry,' he told the cardinals. 'I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only by words and deeds but no less with prayer and suffering.' "

The AP adds that "the last pope to resign was Pope Gregory XII, who stepped down in 1415 in a deal to end the Great Western Schism among competing papal claimants.

Benedict has led the church since 2005. He succeeded Pope John Paul II.

The Vatican is expected to hold a conclave of cardinals to elect a new pope by mid-March.

-- "Immediately, Benedict's decision has both won wide praise as a responsible and humble act and raised a whole rafter of questions. Chief among them: What exactly will be the role of a retired pope?" (National Catholic Reporter)

Update at 12:25 p.m. ET. President Obama Reacts:

"On behalf of Americans everywhere, Michelle and I wish to extend our appreciation and prayers to His Holiness Pope Benedict XVI<' President Obama says in a statement released by the White House. "Michelle and I warmly remember our meeting with the Holy Father in 2009, and I have appreciated our work together over these last four years. The Church plays a critical role in the United States and the world, and I wish the best to those who will soon gather to choose His Holiness Pope Benedict XVI's successor."

Update at 9:35 a.m. ET. After Benedict, "More Continuity Than We'll See Change":

On Morning Edition earlier, Rev. Thomas Reese of Georgetown University said that because Benedict has appointed about 55 percent of the cardinals who will select the next pope (and because the rest of the cardinals were chosen by Pope Paul II), "I think we'll see a lot more continuity than we'll see change" after a successor is chosen.

"After all, the pope did exactly what you or I would do if we were pope," Reese said. He appointed cardinals who agree with him on major issues.

Update at 8:25 a.m. ET. More About Benedict And WWII:

As Sylvia also reports, "in his early teens, at the height of WWII, [Benedict] joined the Hitler Youth — when membership was mandatory. ... Benedict rarely spoke publicly about his childhood during Nazism, or the Catholic Church's relations with the Third Reich. [But] on his first foreign trip as pope, to his native German, he visited a synagogue and addressed what he called the often painful history of relations between Christians and Jews in Germany."

During that visit, the pope called Nazi attempts to exterminate Jews "an insane racist ideology, born of neo-paganism."

-- "During the last months of [World War II] he was enrolled in an auxiliary anti-aircraft corps."

-- "From 1946 to 1951 he studied philosophy and theology in the Higher School of Philosophy and Theology of Freising and at the University of Munich. He received his priestly ordination on 29 June 1951."

-- Named archbishop of Munich on March 25, 1977 and became a cardinal later that year.

"Pope Benedict XVI is at the center of a mounting scandal over pedophile priests, leading to what the weekly National Catholic Reporter calls 'the largest institutional crisis in centuries, possibly in church history.'

"The scandal could have an impact on the pope's legacy. When elected pope, Benedict was not an outsider. He had spent nearly a quarter of a century as the Vatican's top enforcer of doctrine."

Meanwhile, The Associated Press says that the pope's brother, Rev. Georg Ratzinger, says the pontiff's age "is weighing on him. ... At this age my brother wants more rest." A Vatican spokesman has also told reporters, according to Reuters, that the pope made his decision in the "last few months."

Update at 7:23 a.m. ET. The Pope's Statement:

"Dear Brothers,

"I have convoked you to this Consistory, not only for the three canonizations, but also to communicate to you a decision of great importance for the life of the Church. After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry.

"I am well aware that this ministry, due to its essential spiritual nature, must be carried out not only with words and deeds, but no less with prayer and suffering. However, in today's world, subject to so many rapid changes and shaken by questions of deep relevance for the life of faith, in order to govern the bark of Saint Peter and proclaim the Gospel, both strength of mind and body are necessary, strength which in the last few months, has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me.

"For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of Bishop of Rome, Successor of Saint Peter, entrusted to me by the Cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a Conclave to elect the new Supreme Pontiff will have to be convoked by those whose competence it is.

"Dear Brothers, I thank you most sincerely for all the love and work with which you have supported me in my ministry and I ask pardon for all my defects. And now, let us entrust the Holy Church to the care of Our Supreme Pastor, Our Lord Jesus Christ, and implore his holy Mother Mary, so that she may assist the Cardinal Fathers with her maternal solicitude, in electing a new Supreme Pontiff. With regard to myself, I wish to also devotedly serve the Holy Church of God in the future through a life dedicated to prayer."

"The number popes who may have resigned has been estimated as high as 10, but the historical evidence is limited. Most recently, during the Council of Constance in the 15th century, Gregory XII resigned to bring about the end of the Western Schism and a new pope was elected in 1417. Pope Celestine V's resignation in 1294 is the most famous because Dante placed him in hell for it.

"Most modern popes have felt that resignation is unacceptable. As Paul VI said, paternity cannot be resigned."